What you mention is a manual mechanism which is not often used. Instead, developers say:

Requires: python

The automatic mechanism in place is to run ldd, identify the list of libraries used by the executables distributed, and mark those libraries as dependencies. Since the package is build on your newer, Debian 5 package, it requires (automatically) the newer libfoo (like python-2.7), while it would have worked just fine with the older libfoo (python-2.5).